Methods of emission tomography, like for example Positron Emission Tomography (PET) or Single Photon Emission Computer Tomography (SPECT) are common methods of functional imaging. During an examination, a weakly radio-active substance is administered to a biological sample or to an examined human subject and its dispersion within the organism is made visible, e.g. using PET. In this way, biochemical and physiological functions of the organism may be imaged. In emission tomography, molecules which are marked by a radionuclide are thereby used as radio-pharmaceuticals. As a result of radioactive decays, high energy photons are finally emitted, the directions and energies of which are registered by a multitude of detectors being annularly arranged about the examined subject. From the registered decay events the spatial distribution of the radio-pharmaceutical within the body is deduced.
By traversing matter, the photons generated during the decay process may lose energy, be scattered or be absorbed. The respective probabilities for these processes depend on the path-length across matter, on the energy of the photons and on the respective absorption coefficient of the matter. As a consequence, PET requires a correction for the attenuation by components which are located in the beam path in order to deduce the actual radiation dose. The correction of the attenuation requires knowledge of the positions of the attenuating structures which are accounted for in the reconstruction of PET imaging data using an attenuation correction map (attenuation map).
The problems of photon attenuation are not limited to PET but concern emission tomography in general. E.g. the already mentioned Single Photon Emission Computer Tomography (SPECT) above is also encompassed.
Different tissue structures in the body of an organism (bone, fat tissue, air in the lung, etc.) play a central role here, since these different structures also have different attenuation coefficients.
Emission tomography on its own has the disadvantage that it supplies only to a minor extent information about the internal structure of the investigated object. As a consequence, it is often combined with an additional modality, e.g. with computer tomography (CT) or magnetic resonance tomography (MRT). Since CT and MRT feature very different advantages, combined PET-CT as well as combined PET-MRT-devices are in use.
The additional modality is typically used to establish the above-mentioned attenuation correction map. In the example using MRT this is technologically demanding, since there is no direct relationship between the MRT imaging data and the attenuation coefficients.
Hence, by way of example, in MRT-PET devices e.g. segmentation methods are employed in order to identify the tissue structures, in order to generate an attenuation correction map, in which the respective attenuation coefficients are assigned to the segmented tissue types. U.S. Pat. No. 8,724,875 B2, for example, describes such a method.
However, also in an MRT scan hardware parts that are difficult to detect or even invisible pose a problem, e.g. the animal support during pre-clinical imaging, which contribute to the attenuation of photon intensity. Up to now, no respective attenuation correction maps could be established by conventional means.
In the state of the art, methods are known to record the attenuation correction maps of the hardware structures inside the sample volume through direct techniques as e.g. CT and to subsequently superimpose them by the attenuation correction map which had been established with MRT data.
The above-cited US 2014/0221817 A1 is concerned with the problem of photon attenuation due to the diverse hardware parts inside the tomograph. It is suggested to identify the hardware parts optically either through a video control system or by pulsed light measurements (Kinect) or by marking the components for example with an RFID transponder in order to subsequently retrieve the respective, pre-assembled attenuation correction map e.g. from a data base. The publication does not disclose how these maps are created, in particular how the components are generated.